The present invention relates to a power converter apparatus for converting alternating current (AC) into direct current (DC) or vice versa and more particularly, to a power converter apparatus in which at least one of input and output is AC, part of the circuit is a three-line DC circuit having positive, neutral and negative polarities, capacitors are connected between the positive and neutral polarities and the neutral and negative polarities, respectively, and one phase of AC is common to the neutral polarity of DC.
Structurally, in a power converter apparatus for converting AC input into DC and then converting the DC into AC output in which one phase of input AC, a neutral polarity of DC and one phase of output AC are common to each other, current in one phase of AC connected to the neutral polarity of DC flows into a capacitor of the DC circuit or flows out of the capacitor, as described in JP-A-5-15171.
Problems encountered in the prior art will be described with reference to FIGS. 7 and 8. A circuit diagram of the prior art power converter apparatus is shown in FIG. 7 and the power converter apparatus as shown is a three-phase AC input type forward converter which receives three-phase AC having first to third phases via a three-phase AC power supply connecting unit 1 and delivers three-line DC of positive, neutral and negative polarities to an external load via a load connecting unit 62. A forward converter has the three-phase AC power supply connecting unit 1, the load connecting unit 62, transistors 5 and 6 connected in series between the positive and negative polarities, diodes 11 and 12 connected in anti-parallel relation to the transistors 5 and 6, respectively, a reactor 2 connected between a connecting node of the transistors 5 and 6 and a first phase of the three-phase AC power supply connecting unit 1, transistors 9 and 10 connected in series between the positive and negative polarities, diodes 15 and 16 connected in anti-parallel relation to the transistors 9 and 10, respectively, a reactor 4 connected between a connecting node of the transistors 9 and 10 and a third phase of the three-phase AC power supply connecting unit 1, and two capacitors 17 and 18 connected in series between the positive and negative polarities with their node being connected to a second phase of the three-phase AC power supply connecting unit 1 and the neutral polarity. Namely, the forward converter serves as a power converter apparatus which has two half-bridge type converting circuits (also called single-phase forward converters of half-bridge type) and converts three-phase AC into three-line DC. In the following description, the three-phase AC power supply connecting unit 1 is simply referred to as a three-phase AC power supply 1 and the load connecting unit 62 as a load unit 62.
By on-off controlling the transistors 5, 6, 9 and 10, an operation can be ensured in order that an AC input current has a waveform which is ideally a sine wave while being synchronous and in phase with input voltage to have a power factor of 1 and a converted output voltage is rendered to be constant. But to establish this state, completely balanced three-phase AC must be generated in the two half-bridge type single-phase forward converter.
When sine wave current of balanced three phases is supplied from the three-phase power supply, supplied power is temporally constant power. Accordingly, a terminal voltage v.sub.dc across the two series-connected capacitors 17 and 18 in FIG. 7 is complete DC removed of ripple. On the other hand, AC current flowing out of the second phase of the three-phase AC power supply 1 totally flows into the capacitors 17 and 18, provided that no current flows to the neutral polarity from the load unit 62. Accordingly, ripple voltages of the same frequency as that of the AC power supply are caused in terminal voltages v.sub.C1 and v.sub.C2 across the respective capacitors 17 and 18 unless the electrostatic capacitance of the capacitors 17 and 18 is infinite.
An internal waveform in the power converter apparatus of FIG. 7 is shown in FIG. 8, indicating that a ripple as shown in FIG. 8 is involved in the relation among the aforementioned v.sub.dc, v.sub.C1, and v.sub.C2.
More specifically, the ripple voltage can be calculated as follows. It is now assumed that a sine wave current having a peak value I flows out of the second phase of the three-phase AC power supply 1. Inflow electric charge Qx during a half cycle T/2 of power supply period T is determined by integrating a positive half cycle of the sine wave having period T and amplitude I to provide a relation of Qx=T.multidot.I/.pi..
The Qx is a difference between electric charges stored in the capacitors 17 and 18. Accordingly, given that each of the capacitors 17 and 18 has the same electrostatic capacitance which is C, C.multidot.v.sub.C1 -C.multidot.v.sub.C2 =Qx holds.
Given that the difference (V.sub.C1 -V.sub.C2 =.DELTA.V) between terminal voltages of the capacitors 17 and 18 is .DELTA.V, C.multidot..DELTA.V=Qx holds and accordingly, a relation of .DELTA.V=(T.multidot.I)/(.pi..multidot.C) can be obtained.
For example, when AC is of 200 V, 50 Hz and 10 kVA and capacitors of 10 mF are used as the capacitors 17 and 18, T=20 ms and I=40.8 Ap hold and .DELTA.V=26.0 Vpp is obtained from the above relation.
With the change in the neutral point of DC as above, a phase shift and waveform distortion are caused in AC side voltage of the converter even if the transistors 5, 6, 9 and 10 are supplied with an on/off signal resulting from pulse-width modulation (PWM) of a complete sine wave, raising a problem that the input current phase is shifted and the input current waveform is distorted. Further, as .DELTA.V increases, there occurs a time interval during which necessary DC voltage is not obtained, with the result that the AC side voltage of the forward converter cannot reach a peak to cause a time interval during which control is invalidated. Then, the distortion in the input current waveform increasingly grows.
By increasing the capacitance of the capacitors 17 and 18, the neutral point is rendered to be stable and the above problems can be solved. However, mounting of capacitors of large capacitance leads to problems that the cost and the size are raised.